In an internal combustion engine, the tappet is a well-known device and is also commonly referred to as a lifter or valve lifter. For examples of common forms of tappets, see “Automotive Mechanics” (10th Ed.) by William H. Crouse and Donald L. Anglin, McGraw-Hill (1993), ISBN 0-02-800943-6 at pp. 131 and 169–170; and “Power Secrets” by Smokey Yunick and Larry Schrieb, S-A Design Books (1989), ISBN 0-931472-06-7 at pp. 76–80. U.S. Pat. Nos. 5,445,119; 5,638,783; and 5,682,849 to Regueivo, and U.S. Pat. Nos. 5,860,398 and 5,947,069 to Koerner. Each of these documents is hereby incorporated by reference.
In a typical push rod engine, the lifter or tappet generally interacts directly with a rotating camshaft in the engine's valve train. That interaction begins the chain of events that converts the rotary motion of the camshaft into the reciprocating motion of the engine's intake and exhaust valves. The amount of horsepower generated by an engine is related to how efficiently the valve train operates, and thus adjustments to the valve train may have a significant impact on increasing horsepower. In general, the more efficiently air enters and combusted gas exits an engine, as controlled by the opening and closing of the intake and exhaust valves, the more horsepower the engine will produce. “Lifting,” or opening the valves as high and as fast as possible, and closing the valves as fast as possible, is necessary to obtain efficient air and gas flow, and to achieve optimum horsepower. “High lift” is generally obtained by designing a camshaft having aggressive cam lobes with steep flank angles. Consequently, in high-performance applications, a tappet must be able to reliably negotiate the contour of an aggressive cam lobe at extremely high rpm's. In addition, the tappet must be durable and capable of withstanding extreme frictional forces and high valve spring pressures.
Push rod-type internal combustion engines typically use one of four types of tappets or lifters: the flat mechanical tappet, the mushroom tappet, the roller tappet, or the hydraulic tappet. The single piece, flat mechanical tappet is inexpensive, simple to produce, and reliable in stock environments. The mushroom tappet was developed in an effort to address some of the limitations of the standard mechanical tappet, particularly for use with aggressive camshaft designs. The mushroom tappet uses a foot with a larger diameter than the body of the tappet, which allows it to more easily negotiate the steeper flank angles of aggressively designed cam lobes. The roller tappet was developed in large part to overcome the many disadvantages of the mechanical tappet. Roller tappets reduce friction between the cam lobe and lifter foot, thereby reducing lubrication requirements. Thus, roller tappets are desirable in high performance applications, as they can maintain valve train stability at high rpm's and aggressive camshaft designs. Hydraulic lifters have several advantages over both mechanical lifters and roller lifters. Hydraulic lifters automatically compensate for any clearance changes caused by temperature variation or wear. Thus, they should never need adjustment. Also because there is no clearance between the lifter foot and the cam lobe, hydraulic lifters are extremely quiet while in operation when compared to both mechanical or solid lifters. Mechanical or roller lifters need to have some clearance or “lash” between the lifter foot and the cam lobe to act as a cushion to allow for any tolerance changes due to thermal expansion or contraction encountered during repeated engine cycles.